Spectroscopic Identification of a Proto-cluster at Z = 2.300: Environmental Dependence of Galaxy Properties at High Redshift

We have discovered a highly significant over-density of galaxies at z = 2.300± 0.015 in the course of a redshift survey designed to select star-forming galaxies in the redshift range z = 2.3± 0.4 in the field of the bright z = 2.72 QSO HS1700+643. The structure has a redshift-space galaxy over-density of δ g ≃ 7 and an estimated matter over-density in real space of δm ≃ 1.8, indicating that it will virialize by z ∼ 0 with a mass scale of ≃ 1.4× 10 M⊙, that of a rich galaxy cluster. Detailed modeling of the spectral energy distribution– from the rest-far-UV to the rest-near-IR – of the 72 spectroscopically confirmed galaxies in this field for which we have obtained Ks and Spitzer/IRAC photometry, allows for a first direct comparison of galaxy properties as a function of large-scale environment at high redshift. We find that galaxies in the proto-cluster environment have mean stellar masses and inferred ages that are ∼ 2 times larger (at z = 2.30) than identically UV-selected galaxies outside of the structure, and show that this is consistent with simple theoretical expectations for the acceleration of structure formation in a region that is over-dense on large scales by the observed amount. The proto-cluster environment contains a significant number of galaxies that already appear old, with large stellar masses (> 10 M⊙), by z = 2.3. Subject headings: cosmology: observations — galaxies: evolution — galaxies: high-redshift — galaxies: clusters